Psilotaceae relate to their sister family Ophioglossaceae by a reduction of the complex leaf of Ophioglossaceae. However, two scenarios for this reduction are possible: 1) The aerial frond of Psilotaceae corresponds to an over-ground stem, and the leaves have been reduced to small scales along the stem, bearing a sporophore each in their axil, corresponding to the position of the sporophore in Ophioglossaceae. 2)The aerial frond of Psilotaceae corresponds to a leaf, and the sporophores grow along the rachis in the axils of leaflets. The second scenario implies a change of developmental programming between Psilotaceae and Ophioglossaceae, and is the preferred for reasons to be explained.

Broader Impacts: Molecular phylogeny has increased our understanding of evolutionary relations, but also risen questions regarding the evolution of the non molecular features that are actually the target of selection, namely morphology and anatomy. Evolutionary Development (EvoDevo) addresses these questions, linking molecular evolution with the morphological and anatomical developmental pathways of organisms. Namely in old lineages, great gaps have opened between molecular and morphological analysis, probably because of the long evolutionary history and extinctions. Among the ferns (Monilophytes), eusporangiate ferns represent ancestral stem group, but have lost many taxa. The two sister families Ophioglossaceae and Psilotaceae are particularly enigmatic. This study can shed light onto one of its biggest mysteries: the evolution of leaves in ferns and seed plants.